This problem is solved according to the invention in that the resistor has a fixed length and extends between two end connectors fixed thereto and that the tapping consists of two relatively adjustable arms which electrically bridge the resistor section therebetween.
In this case, a resistor of fixed length can be used, for example in the form of a resistor track of thick film. Since the end connectors are fixed, a good electrical connection is obtained. The end connectors may also be provided on a plate. By using the two arms, the resistor section bridged thereby becomes ineffective. The two effective resistor sections accurately correspond to the displacement of the actuating drive. The expense is low because the resistor can be fixed to the one actuating drive part and only the tapping has to be duplicated. It is particularly favourable if abutments for the arms are provided in the region of the end connectors and these are connected by way of a slip clutch to the associated actuating drive part. When the actuating drive is on first operation successively moved to its two limiting positions, the arms are automatically brought into their correct relative position by the abutments.
The abutments may be formed by limiting switches. They can serve to switch the actuating drive off in the limiting positions so that overloading is avoided.
In a preferred embodiment, with an actuating drive having a rotatable output shaft the arms are rotatably displaceably secured thereon and the resistor is fixed on a circle concentric with the output shaft. In this case, the resistor may be secured to the part of the actuating drive that is fixed with respect to the housing. The output shaft need merely be provided with the two arms.
The arms may be resiliently clamped onto the output shaft. They will slide on the output shaft when, on first operation, they are pressed by the actuating drive against the associated abutment.
It is also favourable if a short-circuit track is provided concentric with the resistor, both arms being in electrical contact with the track. By means of this short-circuit track, one ensures that the resistor section between the arms is indeed short-circuited and the measurement will not include any resistances in the region of the clip clutch. Further, the contact arms may even be electrically insulated from the output shaft and, for example, be of plastics material except for the electric contacts.
The measurement will be particularly accurate if the two resistor sections extending between each arm and each end connector are each in series with a drop resistor, both series circuits are connected in parallel and the parallel circuit is in series with a source of constant current, and a measuring voltage can be derived at one of the drop resistors. By reason of the subdivision into two parallel branches of which the resistance is altered in opposite senses, compartively large changes are obtained in the measuring voltage.
In this construction it is advisable for the drop resistors to be much smaller than the potentiometer resistor.